In recent wellbore drilling operations, the drilling is mostly performed in highly deviated and horizontal wellbores. To drill a wellbore as planned prior to drilling, it is important to monitor an inclination of the wellbore and continually determine the position and direction of the drilling tool during drilling. For this monitoring, azimuth with respect to drilling direction and then an axis of the drilling tool is one of important information during drilling. The azimuth can be measured by utilizing some sensors such as a gyro sensor installed in the drilling tool during drilling. In wireline logging operations, a logging tool is conveyed into a wellbore after the wellbore has been drilled. The gyro sensor is used to measure azimuth with respect to the direction of the logging tool.
To improve accuracy and efficiency of the azimuth measurements, a plurality of gyro sensors with each input axis orthogonal to each other may be used. In this combination of the gyro sensors, each gyro sensor is rotated about its rotation axis perpendicular to the input axis. The drive unit for rotating the gyro sensors is configured so as to rotate the gyro sensors stably while maintaining a predetermined angular relationship between the input axes of gyro sensors. In practical point of view, the gyro sensors and the drive unit are installed in relatively narrow space in the foregoing drilling tool and wireline logging tool. Therefore, there is a need for a compact apparatus for azimuth measurements using gyro sensors that can allow the gyro sensors to be stably rotated in cooperation with each other even if such gyro sensors are used, for example, in oilfield and any other harsh environment.